Variable pulse width alarm network



July 9, 1968 v I .B. J. GRACE ,VARIABLE PULSE WIDTH ALARM NETWORK vFiled-Feb. 6, 1964 INVENTOR Burr A. Game 1 d lllllLllwll m mm ATTORNEYSUnited States Patent 3,392,374 VARIABLE PULSE WIDTH ALARM NETWORK BillyJ. Grace, Eau Gallie, Fla., assignor to Radiation Incorporated,Melbourne, Fla., a corporation of Florida Filed Feb. 6, 1964, Ser. No.343,050 6 Claims. (Cl. 340167) ABSTRACT OF THE DISCLOSURE A system formonitoring conditions along a line, such as a pipeline, sewage line,transmission line, and so fort-h, includes a detector responsive toabnormal line conditions for generating a pulse of predeterminedduration indicative of the abnormal condition. Several detectors may beused at different points along the line, in which event each ischaracterized by the generation of a pulse of different duration todistinguish the various monitoring points. At the remote central stationthe incoming pulses on the signal transmission path from the detectingstations are checked for duration, and if of the predetermined pulsewidth, are effective to energize a non-linear device from its normallynon-conducting condition to a conducting state by which an alarmindicative of malfunction or other abnormal condition is actuated. Thealarm remains in the actuated condition by virtue of the biasing of thenon-linear device in its conducting state, once actuated, with a holdingcurrent whose source may be selectively disabled to extinguish thealarm.

The present invention relates to alarm systems and more particularly toa system utilizing variable width pulses that do not require highfrequency communication lines.

Many systems have been developed to monitor conditions at unattended,remote locations, such as at points along a pipe, material transmissionline or in a sewer system. Ideally, indications of these conditions aretransmitted via a readily available low frequency telegraphy or voicefrequency line. In the past, these indications have often beentransmitted as frequency modulated waves. Such waves require atransmission line of relatively wide band width that is beyond thecapabilities of a telegraph line. To avoid this problem, the presentinvention contemplates an alarm system including a plurality of variablepulse width sensing stations that are coupled via a telegraphy line orother medias to plural pulse width sensing networks at a centralstation. When a condition indicative of a malfunction occurs at thesensing station, a low duty cycle pulse of predetermined length istransmitted via the line to the central station. The pulse is of lowduty cycle so that the low frequency telegraph line may be utilizedwithout significant information loss. The pulse width sensing networksat the central station are arranged to activate separate alarms whenpulses of widths commensurate with malfunctions of the monitoredstations occur. Sensing networks at the central station and themonitoring locations are provided on a one for one basis to provideindications of the exact points where the malfunctions occurred.

Each network at the central station includes a differentiator responsiveto the received pulses. The ditferentiator output is applied in parallelto an AND gate and a transistorized monostable multivibrator that isresponsive to only the leading edges of the received pulses. Apredetermined period of time after the multivibrator is activated by theleading pulse edge, the multivibrator generates a time indicating pulsethat is supplied to the AND gate. If the time indicating pulse occurssimultaneously with trailing edge of the received pulse, the AND gate isenable-d. The AND gate output is coupled to an alarm circuit thatincludes a negative impedance device, such as 3,392,374 Patented July 9,1968 a four layer diode. The negative impedance device is activated intoconduction in response to the AND gate output. This device, onceactivated, energizes an alarm circuit even after the AND gate output isdisabled. Thereby, the alarm produces a malfunction indication until amanually activated reset button is energized to block current throughthe negative impedance device.

A feature of the invention is that the transistorized monostablemultivibrator is switched from a fully cut off to a fully conductivecondition in the relatively short time duration of seven microseconds.This is important inthe present system where precisely timed pulses mustbe generated after a first pulse is received. To attain this high speedswitching action, a pair of complementary transistors are connected to aresistance-capacitor charging circuit. During the charging portion ofmultivibrator operation, the resistance-capacitance circuit is coupledbetween the collector and base of the two transistors via a relativelylow impedance path. Once the capacitor attains a predetermined voltage,both transistors are cut off and the capacitor discharges through acircuit that is completely de-c0upled from both emitter-base junctions.

It is an object of the present invention to provide a new and improvedlow band width alarm system utilizing variable width pulses asindicators.

Another object of the invention is to provide a pulse width detectoremploying a monostable multivibrator activated by the leading edge of areceived pulse that is adapted to derive very precisely timed pulsesoccurring after the received pulse since it is switched from a fully cutoff to a completely conducting condition in a relatively short time.

A further object of the invention is to provide a monostablemultivibrator activated by the leading edge of a received pulse, whichmultivibrator is adapted to derive very precisely timed pulses occurringafter the received pulse since it is switched from a fully cut oil to acompletely conducting condition in a relatively short time.

Still another object of the invention is to provide a pulse widthdetector wherein a short duration impulse is generated when the trailingpulse edge coincides with a time indicating signal derived apredetermined time period after the pulse leading edge, and an alarmcircuit is activated in response to said impulse, said alarm circuitremaining activated even after the impulse has terminated.

The above and still further objects, features and advantages of thepresent invention will become apparent upon consideration of thefollowing detailed description of one specific embodiment thereof,especially when taken in conjunction with the accompanying drawing,wherein:

The single figure is a diagram of one preferred embodiment of theinvention.

Referring to the figure, a sewer pipe or material transmission pipe line1 is monitored at a plurality of locations by pressure sensitivetransducers 2, 2 which may be piezo-electric crystals or variablereluctance devices. The output of each transducer 2, 2 is coupled to aseparate variable pulse width generator 3, 3. In re sponse to variationsin pressure at the point where transducer 2 is located, the transducerreactance is varied to control the width of the pulses deriving fromgenerator 3 in a known manner. The duration of the pulses produced bygenerators 3, 3' may vary between microseconds and 300 milliseconds butthe pulses produced by each generator must lie within a predeterminedlimit. For example, the pulses deriving from generator 3 may varybetween 100 microseconds and 30 milliseconds while those deriving fromgenerator 3 may vary between 60 milliseconds and 300 milliseconds. Theduty cycle of each wavetrain supplied to line 4 is quite low, i.e. theperiod between adjacent pulses is on the order of a few seconds, so thata low frequency line can be utilized.

The outputs of generators 3, 3' are applied in parallel to telegraphyline 4. At a central monitoring station, a plurality of detectingcircuits 5, are provided. Each detecting circuit includes means foractivating an alarm when a pulse of predetermined width is transmittedto it via line 4.

In operation, pulse generator 3 is constructed so that a pulse of firstpredetermined width is generated by it when the pressure sensed bytransducer 2 reaches a level indicative of a fault in pipe line 1. Whena fault occurs in pipe line 1 at the point where transducer 2' islocated, generator 3' supplies a pulse of a second predetermined widthdifferent from the first width to line 4. At the monitoring station,sensing circuits 5 and 5' are adjusted to activate alarms when pulses ofthe first and second widths are transmitted on line 4.

Each of the sensing circuits 5 and 5 is identical except for the valuesof impedances employed in a timing circuit. Circuit 5 includes adifferentiator including capacitor 11 and resistor 12 that is responsiveto the variable width polar telegraph pulses transmitted on line 4. Thepulses have negative and positive leading and trailing edges,respectively. The negative going, leading pulse edge is coupled throughcapacitor 11 as a short duration, negative pulse. The negative pulsederiving from differentiator 10 is coupled to the input of monostablemultivibrator 13 via blocking diode 14 and current limiting resistor 15.

Monostable multivibrator 13, which derives a pulse a predeterminedperiod of time after a negative input is applied thereto, includescommon emitter PNP transistor 16 and NPN transistor 17. Thecollector-emitter path of transistor 16 is energized by the negative DC.voltage at terminal 18 via collector load resistor 19 and emitter biasresistor 21. The emitter of transistor 16 is maintained at a negativebias by the negative voltage applied thereto from terminal 22 via diode23 and currently limiting resistor 24. The base of transistor 16 isnormally maintained at zero potential by the path including resistor 25and series connected diodes 26 and 27, which diodes are connected toshunt positive current away from the transistor 16 base electrode. Sincethe emitter of transistor 16 has a quiescent negative bias relative toits base, the transistor is normally maintained at out OK.

Because no current normally flows to the collector of transistor 16 viaresistor 19, the base of transistor 17 is maintained back biased, underquiescent conditions. Back biasing results since the negative voltagecoupled from terminal 18 to the transistor base is greater than thenegative voltage coupled from terminal 22 to the transistor emitter viadiode 23.

Connected in parallel with the collector of transistor are a pair ofparallel paths, the first consisting of resistor 28 and the second of aseries circuit including variable timing resistor 31, timing capacitor32 and diode 26 and diode 27.

To generate short duration pulses in response to sudden voltagetransitions developed across resistor 28, a further differentiator 33including capacitor 34 and resistor 35 is provided. The output ofdiflerentiator 33 is coupled to one input of reverse biased diode ANDgate 36, the other input of which is responsive to the output ofditferentiator 10. Gate 36 includes diodes 37 and 38 having theircathodes connected to the outputs of ditferentiators 33 and 10,respectively, and to the negative voltage at terminal 39, the latterconnections being through biasing resistors 41 and 42. Positive voltageis applied to the anodes of diodes 37 and 38 from terminal 43 viacurrent limiting resistor 44. The output of gate 36, at the anodes ofdiodes 37 and 38, is developed across load resistor 45. Normally, diodes37 and 38 conduct lightly so that a large positive voltage is maintainedacross load resistor 45 under quiescent conditions.

Shunting resistor 45 is a series circuit including four layer diode 46,alarm transducer 47, such as an audio and/or visual current activateddevice, manually activated,

normally closed switch 48 and the positive voltage at terminal 49. Underquiescent conditions, the positive voltage applied to resistor 45 fromterminal 43 is such that the anode cathode voltage of four layer diode46 is less than the diode firing voltage.

In operation, the negative going, leading edge of the pulse applied tothe input of differentiator 10 is shaped into a short duration negativepulse by the difierentiator. The negative pulse is applied to the baseof transistor 16 via diode 14 and resistor 15 to trigger the transistorcollector into conduction. The fiow of current through theemitter-collector path of transistor 16 forward biases the base oftransistor 17 In response to transistor 17 being forward biased,negative cur-rent flows from terminal 22 to ground through the seriespath consisting of diode 23, the emitter-collector path of transistor17, resistor 31, capacitor 32, diode 27 and resistor 25. The resultingnegative voltage across resistor 25 maintains the base of transistor 16forward biased even after the short duration pulse from differentiator10 has subsided. In consequence, transistor 17 remains forward biasedand the current path through resistors 25 and 31 is maintained. Becauseof the high impedance input circuit of transistor 16 reflected todifferentiator 10 and the low impedance regenerative circuit from thecollector of transistor 17 to the base of transistor 16, monostablernultivibrator 13 is triggered from cutoff to a fully conductingcondition in the relatively short time of seven microseconds. Thisenables the trailing edge of the pulse deriving from monostablemultivibrator 13 to be generated at a precisely determined instant.

As current continues to flow through capacitor 32, charge is built up onthe capacitor so that the plate conr nected to diode 27 is positiverelative to the plate connected to resistor 31. As charge builds up oncapacitor 32, current through the series path including resistor 25decreases. Ultimately, the current decreases to the point where thenegative voltage across resistor 25 is less than the negative voltage atthe emitter of transistor 16. When this occurs, collector current fortransistor 16 is cut off, the voltage at the base of transistor 17decreases to the voltage at terminal 18 and collector current throughthe latter transistor also ceases. In consequence, the current path fromterminal 22 through transistor 17 and resistors 25 and 31 no longerexists and the voltage at the base of transistor 16 drops suddenly tozero. When the base voltage of transistor 16 drops to zero, thattransistor as well as transistor 17 become more effectively cut off in aregenerative manner.

With transistor 17 cut olf, the charge accumulated by capacitor 32 isdissipated through the series path consisting of diode 26 and resistors28, 31. The impedance of resistor 28 is considerably larger than thecombined impedances of resistor 31 and diode 26. As a result, most ofthe voltage that existed across capacitor 32 when transistor 17 was cutoff will be suddenly reflected as a large negative going voltage acrossresistor 28, i.e. the voltage at the junction between resistor 28 andcapacitor 34 suddenly drops to a voltage less than round. The charge ofcapacitor 32 is quickly reduced substantially to zero at a ratedetermined by the impedances of resistors 28, 31 and diode 26, as wellas the capacitor value. As the capacitor charge is dissipated, thevoltage across resistor 28 rises exponentially to zero.

The voltage waveform across resistor 28 when transistor 17 is conductingis essentially a negative going step of low amplitude. The leading edgeof the step occurs simultaneously with the transistors being driven outof cut off. Since transistor 17 is essentially a constant impedance whenconducting and resistor 28 is decoupled from capacitor 32 by diode 26,the current through, hence voltage across the resistor 28 remains at aconstant value, equal to the step, while the transistor is conducting.It is a low amplitude step because the combined impedance of transistor17 and diode 23 is considerably less than the impedance of resistor 28.

It is thus seen that the voltage derived from dilferentiator 33 includesa low voltage negative pulse occurring simultaneously with the leadingedge of the pulse applied to differentiator 10 followed by a largenegative pulse that is generated a predetermined time after the lowvoltage pulse. The second pulse is generated at a time controlled by thevalue of resistor 31 which determines when transistors 16 and 17 will beagain driven to cutolf.

AND gate 36 is biased such that the low negative voltages applied todiodes 37 and 38 by dilferentiators 33 and 10, respectively, in responseto the leading edge of the system input pulse will not reduce thevoltage across load resistor 45 to a value where four layer diode 46will be activated. If the large negative pulse deriving fromdifferentiator 33 does not occur simultaneously with the trailing edgeof the system input pulse, a conduction path exists for it throughforward biased diodes 37 and 38 to ground via resistor 42 and diode 14.This occurs since the ditferentiator output is substantially at groundpotential except when the leading and trailing edges of the system inputpulse occur. In consequence, a significant portion of the negative pulseis not developed across load resistor 45 and the anode-cathode voltageof four layer diode 46 is maintained below the diode firing level.

Only when the trailing, positive going edge of the input pulse and thelarge negative pulse generated by monostable multivibrator 13 occursimultaneously will the latter voltage have any effect on diode 46. Atsuch time, the positive voltage pulse applied to the anode of diode 38by ditferentiator 10 cuts off the diode and substantially all currentfrom the large negative coupled to diode 37 is fed to load resistor 45.In consequence, the voltage diflierence between the anode and cathode ofdiode 46 increases to a point beyond the diode firing level. Once thefiring level of diode 46 is achieved it remains conducting even thoughits anode-cathode voltage drops to a level less than the firing level.This is because four layer diodes are negative impedance devices thatdraw current once their firing level is attained. The amount of currentdrawn is a function of the value of resistor 45, which in this caseallows holding current to flow through diode 46.

With diode 46 conducting heavily, alarm 47 is activated to provide anindication that a pulse having a predetermined width, indicative of pipeline or sewer system failure, has been received. The alarm remainsenergized until reset button is pressed to terminate the flow of currentthrough alarm 47 and diode 46. The detector is now reset to its originalcondition, ready to provide an indication of pipe line or sewer systemfailure. If the pulse durations for the various lines being monitoredare different, the present invention is adapted to produce the secondpulses at variable times by adjusting the value of resistance 31 in thetiming circuit for monostable multivibrator 13.

While I have described and illustrated one specific embodiment of myinvention, it will be clear that variations of the details ofconstruction and transmission media which are specifically illustratedand described may be resorted to without departing from the true spiritand scope of the invention as defined in the appended claims.

What I claim is:

1. A monitoring system for indicating the existence of a line conditionof predetermined character at a station remote from points at which lineconditions are being observed, said system comprising a transmissionpath, means responsive to said line condition of predetermined characterfor generating pulses of predetermined duration less than the intervalbetween pulses on said transmission path; and, at said station, displaymeans including a monostable multivibrator normally biased to operate inits stable state, differentiating means responsive to transitionsoccurring at the leading and trailing edges of each incoming pulse onsaid path for generating respective short duration pulses in responsethereto, means responsive to the short duration pulse generated inresponse to the leading edge transition of an incoming pulse fortriggering said multivibrator to its unstable state, said multivibratorincluding means for controlling the duration of time said multivibratorremains in said unstable state upon being triggered thereto, saidduration of time corresponding to said predetermined pulse duration, anon-linear device having a non-conducting state and capable of beingenergized to a conducting condition upon application thereto ofa voltagehaving an amplitude equal to or greater than a predetermined firingvoltage level, means biasing said device to remain in said conductingcondition once energized thereto, means responsive to reversion of saidmultivibrator to said stable state for generating a short duration pulsethereupon, detection means responsive to coincident occurrence of ashort duration pulse generated by said reversion responsive means and ofa short duration pulse generated by said transition responsive means inresponse to the trailing edge transition of the incoming pulse forapplying a voltage of said amplitude to said non-linear device, meansresponsive to assumption of said conducting condition by said non-lineardevice for indicating the occurrence of a pulse of said predeterminedduration, and means for selectively disabling said biasing means toreset said non-linear device to said non-conducting state and thereby toremove said indication.

2. The system according to claim 1 wherein said duration controllingmeans is variable to permit selective alteration of the time intervalover which said multivibrator remains in said unstable state.

3. The system according to claim 1 wherein said nonlinear device is afour-layer diode.

4. The system of claim 1 wherein said means responsive to line conditionincludes a plurality of generating means positioned at different pointsalong the line and each for generating a pulse of differentpredetermined duration signifying the existence of said line conditionof predetermined character at a respective point along the line; andwherein is included at said station a like plurality of said displaymeans, each including a monostable multivibrator with said means forcontrolling duration establishing a duration corresponding respectivelyto that of the pulse to be detected by that particular display means.

5. The system according to claim 1 wherein said means responsive toreversion of said multivibrator to said stable state comprises a furtherdiiferentiator.

'6. The system according to claim 1 wherein said coincident detectionmeans comprises an AND gate including a pair of normally reverse biaseddiodes, each of said diodes responsive to a respective one of said shortduration pulses to assume a state of conduction.

References Cited UNITED STATES PATENTS 2,484,352 lO/1949 Miller et. al.3,122,647 2/1964 Huey. 3,215,852 11/1965 Brode et a1.

JOHN W. CALDWELL, Primary Examiner.

DONALD J. YUSKO, Examiner.

